CROSS REFERENCE TO RELATED APPLICATIONS
This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Patent Application 2000-351658, filed on Nov. 17, 2000, the entire content of which is incorporated herein by reference.
This invention relates to a displacement detecting device sliding a brush on a resistance and generating an output signal based on a rotational portion of the brush. Especially, this invention relates to an arrangement of a circuit pattern of a power supply line, an earth line, and an output line on a contact-type displacement sensor.
Conventionally, this species device (e.g., displacement sensor) includes a resistance coat for serving as resistance on one rotational side (e.g., shaft) and a non-rotational side (e.g., housing) and a brush for sliding on the resistance coat on another side thereof. The brush is fixed with the rotational side (e.g., shaft) via a brush holder, and the brush is rotated according to the movement of a detected object. On a structure such as this, the brush slides on the resistance when supplying a predetermined voltage between end portions of the resistance. When the brush slides on the resistance, as an output voltage of the sensor changes according to the position of the brush, a displaced position of the detected object can be detected. For example, a device such as this is disclosed in Japanese patent laid open No. H09(1997)-311012.
The device (sensor) shown in this publication discloses that two output terminals (a first terminal generating an output voltage VTA, and a second terminal generating an output voltage VTB) outputs a signal output respectively when the predetermined voltage is supplied between a power supply terminal (terminal supplied to a predetermined voltage V1) and an earth terminal (terminal connected with the earth) on a substrate. The sensor includes four terminals, two arc-shaped conductors, a sliding resistance, and a pair of brushes.
The power supply terminal electrically contacts with the earth terminal via an offset resistance and the sliding resistance. One output terminal electrically connects with a conductor and the earth terminal electrically connected with the power supply terminal via the sliding resistance; another output terminal electrically connects with the conductor. Two sliding members (two brushes) are fixed on a shaft side rotating with the detected object on this construction. The brushes slide on the sliding resistance and the conductor simultaneously, and the brushes are electrically connected with the sliding resistance and the conductor. Thus, the signal output according to a position of the detected object (e.g., a rotational position of the brush) is generated from two output terminals. However, two arc-shaped output patterns are formed at both sides of the shaft on the above-mentioned construction, and a power supply line and an earth line are formed at the both sides. Therefore, when an electrical short happens on the output line formed by a conductor for whatever reason, a failure occurs wherein the output terminal outputs half of the supplied voltage for the terminal supplying a power source. The half of the supplied power voltage does not change even if the position of the brush was changed. Therefore, when the failure occurs, under the construction outputting half of the supplied voltage, the controller controlling the output signal treats the generated voltage from the output terminal as an output signal of a normal condition even if the failure occurs.
In view of the foregoing, it is an object of the present invention to provide a construction which detects a failure even if a short between the output terminals occurs.
According to an aspect of the present invention, a displacement detecting device includes a power supply line made of a conductor on a substrate, an earth line made of a conductor on the substrate, a plurality of output lines made of a conductor on the substrate, a resistance formed between the power supply line and the earth line on the substrate, and a brush sliding on the resistance and at least one of the output lines and electrically connected between the resistance and at least one of the output lines. The displacement detecting device generates the output signal from the output lines based on the position of the brush, wherein one of the power supply line and the earth line is disposed between the output lines.
Therefore, even if a plurality of output lines are disposed adjacent to each other, one of the power supply line and the earth line is formed between nearby output lines. For whatever reason, when an electrical short happens between the output lines, the voltage of the output line comes to the predetermined voltage (supplied voltage) supplied for the sensor or the earth voltage (e.g., 0V). Therefore, when the electrical short between the output terminals happens, it is able to detect the failure.